In those days: i.e. Hezekiah's fourteenth year: for fifteen years (603-588 B.C.) are added to his life (Isa_38:5), and he reigned twenty-nine years (2Ki_18:2); 14 + 15 = 29.
sick. This sickness was therefore during the siege.
the LORD. Hebrew. Jehovah.App-4.
Set thine house in order = Give charge concerning thy house.
die, and not live = thou wilt certainly die. Figure of speech Pleonasm: by which a thing is put both ways (positive and negative) for emphasis.